New loudspeaker technologies are being considered for use in mobile products which have a number of advantages over the moving coil types currently being used, such as potentially higher efficiency, higher quality or greater flexibility regarding product form factor. However, what most of these have in common is very light flexible diaphragms and therefore would not work with, e.g., sealed-cavity design paradigm, since this would provide too much stiffness and therefore greatly reduce the low frequency output. An open back design would not be satisfactory either since the sound radiated from the rear would partially cancel the sound radiated from the front because the two are in opposite phase. This appears to be a major technology bottleneck.
Thus currently conventional heavy (moving mass) and inefficient moving coil loudspeakers with sealed back cavities are used in mobile products. Light diaphragms are currently only used in hi-fi loudspeakers using the electrostatic or planar magnetic principles, where the diaphragms can be made large enough to counteract the cancellation effects of the rear wave. So called “sound absorbing” materials are used in non-mobile loudspeaker cabinets to control standing waves, but they have little effect at lower frequencies and therefore do not allow the size of the cabinet to be reduced by very much. Such materials include fibrous materials, foams and other porous materials in which the pores are essentially random in size.